Polyesters have long been proven to be particularly useful polymers.
Curable compositions are employed in the production of potted or encapsulated electronic or electrical components. Both epoxy and unsaturated polyester resin compositions are used for this purpose. Epoxy resins are much more expensive, but are favored. One of the reasons that epoxy resins are favored over unsaturated polyesters is the fact that unsaturated polyesters will exhibit about 5-10% degree of volume shrinkage during cure. When potting electrical or electronic components this shrinkage is undesirable because it will result in separation from the walls of the container (which becomes an integral part of the finished component) or in cracks in the cured resin.
Curable resins are also used in a potting technique known as "pot-on-sand". In this technique, sand is poured into the container to cover the electrical or electronic component and a low viscosity resin is then poured over the sand. This resin must completely penetrate the sand before it cures to give a completely solid void free potted part.
In the past a number of thermoplastics such as polyvinyl acetate, polycaprolactone, polystyrene, polyethylene, styrene-butadiene copolymers and liquid rubbers such as butadiene-acrylonitrile and hydroxy terminated polyepichlorohydrin have been used to reduce the shrinkage of unsaturated polyester resins. All of these examples suffer from one of two problems. Either they are not compatible with unfilled unsaturated polyester resin solutions (and therefore can be used only in highly filled compositions) or else when added to the unsaturated polyester resin they result in greatly increased viscosity.
It has now been discovered that improved unfilled, solution stable, unsaturated polyester resin compositions can be provided by adding thereto a member of a family of specified aliphatic polyols. Such compositions exhibit low shrinkage on cure but still are low enough in viscosity to penetrate fine sand. These compositions are thus uniquely suitable for use in electrical or electronic pot-on-sand applications. As will be seen hereinafter, when poured onto two inches of sand, the resin of this invention completely penetrates the sand before gelation occurs.
The teachings of A. S. Burhans, Insulation/Circuits; January 1979, pages 37, are acknowledged. However, this relates to the use of polypropylene polyols in reactive admixture with cycloaliphatic epoxies to flexibilize them and improve their thermal shock resistance. This in no way makes it obvious to use polypropylene glycols with unsaturated polyesters to reduce shrinkage in the latter.